Cancer stem cells (CSCs) play a critical role in breast cancer progression and chemoresistance. Recently, we found that induction of an epithelial-mesenchymal transition (EMT) in breast cancer cells confers stem cell attributes. We also identified the Forkhead transcription factor FOXC2 as an orchestrator of the mesenchymal program underlying EMT and a key regulator of metastatic competence. Our preliminary studies show that suppression of FOXC2 leads to reversion of EMT and loss of functional stem cell properties. Moreover, FOXC2 expression is higher in stem cells isolated from normal mammary tissues or mammary epithelial cell lines. This apparent positioning of FOXC2 at the crossroads of EMT and breast cancer stem cells suggests that it or its effectors are essential for the sustainment and/or functioning of breast cancer cells with mesenchymal and stem cell properties. In this proposal, we will employ a dual systems approach - an in vitro cell culture model and a Foxc2 conditional knockout mouse - to delineate the pleiotropic functions of Foxc2 in normal breast homeostasis and cancer progression. We will cross Foxc2 conditional knockouts to metastasis-prone transgenics to determine the consequences of Foxc2 ablation on tumor initiation and metastasis and test the hypothesis that Foxc2 is a critical regulator of stem-like attributes and resistance to chemotherapy in vivo. In addition, we have identified PDGFR-B as a putative druggable target downstream to FOXC2 and we will evaluate the efficacy of PDGFR-targeted agents in eradicating FOXC2-expressing EMT-derived CSCs. Thus this project will improve our understanding of the role of CSCs and EMT in breast cancer progression and will suggest putative FOXC2 molecular effectors that may be exploited to target metastatically-competent cells with EMT/CSC attributes. PUBLIC HEALTH RELEVANCE: Metastasis and resistance to chemotherapy are the major causes of breast cancer-related mortality. Cancer stem cells (CSCs) play critical roles in both these processes, and we recently found that CSCs can be generated by an epithelial-mesenchymal transition (EMT), a latent embryonic process that has been shown to promote breast cancer progression. By using in vitro and in vivo models, we will characterize the pleiotropic functions of FOXC2 as a central mediator of EMT, stemness, metastatic competence and drug resistance, and the expected results will suggest possible molecular targets that may be exploited to selectively treat metastatic and resistant breast cancers.